It is generally known that antenna performance is dependent on the size, shape and material composition of the constituent antenna elements, as well as the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna operational parameters, including input impedance, gain, directivity, signal polarization, operating frequency, bandwidth and radiation pattern. Generally for an operable antenna, the minimum physical antenna dimension (or the electrically effective minimum dimension) must be on the order of a half wavelength (or a multiple thereof) of the operating frequency, which thereby advantageously limits the energy dissipated in resistive losses and maximizes the transmitted energy. Alternatively, a quarter-wavelength antenna operating over a ground plane performs similarly to a half-wavelength antenna. Quarter-wavelength and half-wavelength antennas are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth or multiple frequency-band operation, and/or operation in multiple modes (e.g., selectable radiation patterns or selectable signal polarizations). Smaller packaging of state-of-the-art communications devices, such as cellular handsets and personal digital assistants, do not provide sufficient space for the conventional quarter and half wavelength antenna elements. Thus physically smaller antennas operating in the frequency bands of interest and providing the other desirable antenna operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
As is known to those skilled in the art, there is a direct relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna, according to the relationship: gain=(βR)^2+2βR, where R is the radius of the sphere containing the antenna and β is the propagation factor. Increased gain thus requires a physically larger antenna, while communications equipment manufacturers and users continue to demand physically smaller antennas. As a further constraint, to simplify the system design and packaging, and strive for a minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-band and/or wide bandwidth operation, allowing the communications device to access various wireless services operating within different frequency bands from a single antenna. Finally, gain is limited by the known relationship between the antenna frequency and the effective antenna length (expressed in wavelengths). That is, the antenna gain is constant for all quarter wavelength antennas of a specific geometry i.e., at that operating frequency where the effective antenna length is a quarter wavelength of the operating frequency.
The known Chu-Harrington relationship relates the size and bandwidth of an antenna. Generally, as the size decreases the antenna bandwidth also decreases. But to the contrary, as the capabilities of handset communications devices expand to provide for higher data rates and the reception of bandwidth intensive information (e.g., streaming video), the antenna bandwidth must be increased.
One basic antenna commonly used in many applications today is the half-wavelength dipole antenna. The radiation pattern is the familiar omnidirectional donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical antenna gain is about 2.15 dBi. Clearly, such antennas are not acceptable for handheld communications devices.
The quarter-wavelength monopole antenna placed above a ground plane is derived from a half-wavelength dipole. The physical antenna length is a quarter-wavelength, but when operating over the ground plane the antenna performance resembles that of a half-wavelength dipole. Thus, the radiation pattern for a monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
The common free space (i.e., not disposed above a ground plane) loop antenna (with a diameter of approximately one-third an operating wavelength) also displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics to conventional transmission lines. However, conventional loop antennas are too large for handset applications and do not provide multi-band operation. As the loop length increases (i.e., approaching one free-space wavelength), the maximum of the field pattern shifts from the plane of the loop to the axis of the loop. Placing the loop antenna above a ground plane generally increases its directivity.
Printed or microstrip antennas are constructed using patterning and etching techniques common in printed circuit board processing, with an upper metallization layer serves as the radiating element. These antennas are popular because of their low profile, the ease with which they can be fabricated and a relatively low fabrication cost. One such antenna is the patch antenna, comprising in stacked relation, a ground plane, a dielectric substrate and a radiating element. The patch antenna provides directional hemispherical coverage with a gain of approximately 3 dBi. Although small compared to a quarter or half wavelength antenna, the patch antenna has a relatively poor radiation efficiency, i.e., the resistive return losses are relatively high within its operational bandwidth. Disadvantageously, the patch antenna also exhibits a relatively narrow bandwidth. Multiple patch antennas can be stacked in parallel planes or spaced-apart in a single plane to synthesize a desired antenna radiation pattern that may not be achievable with a single patch antenna.
Given the advantageous performance of quarter and half-wavelength antennas, conventional antennas are typically constructed so that the antenna length is on the order of a half wavelength of the radiating frequency or a quarter wavelength with the antenna operated above a ground plane. These dimensions allow the antenna to be easily excited and operated at or near a resonant frequency, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. But, as the operational frequency increases/decreases, the operational wavelength correspondingly decreases/increases. Since the antenna is designed to present a dimension that is a quarter or half wavelength at the operational frequency, when the operational frequency changes, the antenna is no longer operating at a resonant condition and antenna performance deteriorates.
As can be inferred from the above discussion of various antenna designs, each exhibits known advantages and disadvantages. The dipole antenna has a reasonably wide bandwidth and a relatively high antenna efficiency (gain). The major drawback of the dipole, when considered for use in personal wireless communications devices, is its size. At an operational frequency of 900 MHz, the half-wave dipole comprises a linear radiator of about six inches in length. Clearly it is difficult to locate such an antenna in the small space envelope of today's handheld communications devices. By comparison, the patch antenna or the loop antenna over a ground plane present a lower profile resonant device than the dipole, but operates over a narrower bandwidth with a highly directional radiation pattern. Thus placing an antenna proximate the ground plane of a printed circuit board that carries electronic components associated with operation of the communications device degrades performance of the antenna, especially lowering the antenna bandwidth.
As discussed above, multi-band or wide bandwidth antenna operation is especially desired for use with various personal or handheld communications devices. One approach to producing an antenna having multi-band capability is to design a single structure (such as a loop antenna) and rely upon the higher-order resonant frequencies of the structure to obtain a radiation capability in a higher frequency band. Another method employed to obtain multi-band performance uses two separate antennas, placed in proximity, with coupled inputs or feeds according to methods well known in the art. Each of the two separate antennas resonates at a predictable frequency to provide operation in at least two frequency bands, at the expense of consuming a greater volume within the communications handset. Thus it remains difficult to realize an efficient antenna or antenna system that satisfies the multi-band/wide bandwidth operational features in a relatively small physical volume.
The “hand” or “body” effect must also be considered during the design of antennas for handheld communications devices. Although an antenna incorporated into such devices is designed and constructed to provide certain ideal performance characteristics, in fact all of the performance characteristics are influenced, some significantly, by the proximity of the user's hand or body to the antenna when the communications device is in use. When the hand of a person or other grounded object is placed close to the antenna, stray capacitances are formed between the effectively grounded object and the antenna. This capacitance can significantly detune the antenna, shifting the antenna resonant frequency (typically to a lower frequency), thereby reducing the received or transmitted signal strength. It is impossible to accurately predict and design the antenna to ameliorate these effects, as each user handles and grasps the personal communications device differently.
Recently, cellular handsets have been designed to operate in three frequency bands: 824–894 MHz (AMPS/CDMA) or 880–960 MHz (GSM) and 1710–1880 MHz (DCS) and 1850 MHz–1990 MHz (PCS). There is, however, a desire to have a handset antenna capable of functioning anywhere in the world. Ideally, such a handset comprises a single antenna that supports all four frequency bands identified above. In addition, use of one antenna design reduces antenna inventory requirements for different cellular telephones operative in different frequency bands.
Prior art antennas cannot operate in each of the four listed bands. Broadband antennas (providing continuous coverage over all frequency ranges of interest) are too large to be used in most if not all handsets being manufactured in the 2003–2004 period. Thus as can be seen, the various prior art antennas have certain advantageous features, but none offer all the performance requirements desired for handset and other wireless communications applications, including multi-band operation, high radiation efficiency, wide bandwidth, high gain, low profile and low fabrication cost.